Cells normally maintain a balance between protein synthesis, folding, trafficking, aggregation, and degradation, referred to as protein homeostasis, utilizing sensors and networks of pathways [Sitia et al., Nature 426: 891-894, 2003; Ron et al., Nat Rev Mol Cell Biol 8: 519-529, 2007]. The cellular maintenance of protein homeostasis, or proteostasis, refers to controlling the conformation, binding interactions, location and concentration of individual proteins making up the proteome. Protein folding in vivo is accomplished through interactions between the folding polypeptide chain and macromolecular cellular components, including multiple classes of chaperones and folding enzymes, which minimize aggregation [Wiseman et al., Cell 131: 809-821, 2007]. Whether a given protein folds in a certain cell type depends on the distribution, concentration, and subcellular localization of chaperones, folding enzymes, metabolites and the like [Wiseman et al.]. Human loss of function diseases are often the result of a disruption of normal protein homeostasis, typically caused by a mutation in a given protein that compromises its cellular folding, leading to efficient degradation [Cohen et al., Nature 426: 905-909, 2003]. Human gain of function diseases are similarly frequently the result of a disruption in protein homeostasis, such as the accumulation of misfolded proteins, leading to protein aggregation [Balch et al. (2008), Science 319: 916-919].
The proteasome is a large protein complex of multiple subunits which acts as a protease to degrade misfolded proteins. Most proteasome substrates are targeted for degradation by the covalent attachment of ubiquitin moieties which are recognized by the proteasome [Lee et al. (2010), Nature 467(7312): 179-184]. Proteins with longer ubiqutin chains tend to have a stronger association with the proteasome than those with smaller chains [Lee et al. (2010); Proctor et al. (2007), BMC Systems Biology 1: 17]. The length of the ubiquitin chains is modulated, in part, by proteasome-associated deubiquitinating enzymes. One such mammalian deubiquitinating enzyme is Usp14 which has been shown to act as an inhibitor of the proteasome [Lee et al. (2010)].
Both proteasome dysfunction and dysfunction in proteostasis have been implicated in a diverse range of diseases including for example, neurodegenerative disease, metabolic diseases, inflammatory diseases, and cancer. In many such diseases and conditions, the proteasome has decreased ability to degrade misfolded or abnormal proteins, leading to the presence of toxic protein aggregates. In addition, the enhancement of proteasome activity can be therapeutic for any disease characterized by deficient proteasome activity, or deficient activity of other components of the ubiquitin-proteasome pathway including, but not limited to, von Hippel-Lindau disease, spinocerebellar ataxia 1, Angelman syndrome, giant axon neuropathy, inclusion body myopathy with Paget disease of bone and frontotemporal dementia (IBMPFD), and others (Lehman, N. L., (2009), Acta Neuropathologica, 118(3), 329-347; Weihl et al., (2007), Neuromuscular Disorders, 17, 87-87). Enhancing proteasome activity is also therapeutic for diseases in which proteasome substrates are involved and contribute to pathology, but which do not satisfy a strict definition of proteinopathies. For example, numerous oncoproteins are proteasome substrates and their ability to promote cancer can potentially be attenuated by enhancing proteasome activity. Therefore, there is a need for compounds and pharmaceutical compositions to treat conditions associated with proteostasis dysfunction and/or that provide therapies based on enhancing proteasome activity.